The present invention relates to a printhead for an electrooptical plotter and, more particularly, to a printhead that produces a multiplicity of light spots on a surface to be plotted, such as that of a printing plate in a plate imagesetter.
In a laser imagesetter, the light from one or more laser sources is focused onto the surface of a light sensitive film or plate. For brevity, this surface (whether of a film or a plate) will be referred to hereunder as the film. The plotting rate is generally limited by the power of the laser beam and by the speed at which it can be made to sweep the surface of the film. The power limitation is particularly problematic in plate imagesetters, owing to the inherently low light sensitivity of current printing plates. In order to achieve a high plotting rate, it is known to employ a multiplicity of laser sources, usually laser diodes (LDs), operating in parallel and producing a multiplicity of traces on the film when scanning it. A practical device for such a purpose is an individually addressable laser diode array (IALDA), which consists of a single bar of semiconducting material, such as GaAs, in which a linear array of addressable lasing sections has been formed. It is noted that such an IALDA device is distinct from a non-addressable laser diode array device, which has been used in imagesetters as a light source to illuminate an array of light modulators. The advantages of IALDA over an array of discrete LDs are much lower costs and the closer achievable spacing of the individual lasing sections, although they still cannot be contiguous. One possible disadvantage of IALDA is that at high power output any one section may fail, which would render the whole device useless for plate plotting. Recently, however, practical IALDA devices of sufficient power and reliability have become available.
Two important characteristics of any laser diode, or lasing section in an LD array device, particularly in the case of multi-mode LD (which serves for high power applications, such as that addressed by the present invention), are that (1) the light emitting region has a very elongated shape, typically 1 micron across and 50 to 200 microns along, and (2) the beam divergence in the cross direction is relatively high--typically 45 degrees FWHM, corresponding to a numerical aperture (NA) of 0.4--while in the length direction it is relatively low--typically 12 degrees FWHM, corresponding to a NA of 0.1.
In one common type of light projection system of prior art, employed inter alia in imagesetters, the light beams emitted by an IALDA device, or a linear array of laser diodes, is simply focused by an objective lens onto the film, thus forming on the film an array of projected spots, which is the image of the array of lasing regions, as is illustrated in FIG. 1. The film travels in a certain direction 44, causing the light spots to record parallel traces 46 on the film, as shown. Now, if the light emitting regions on the array, and consequently also the projected spots, were essentially contiguous, the array would be oriented so that the length axis 42 of its image is normal to the direction of travel 44. Since, however, the spots are generally not contiguous, the device or the array is usually turned, in a plane parallel to the film, so that length axis 42 becomes oriented by an angle .alpha. with respect to the direction normal to the direction of travel 44, and thus traces 46 become closer together. If the pitch of the spots, i.e. the distance between the centers of adjacent spots, is p, then, clearly, the pitch of the traces will be p'=p*cos .alpha.. The individual laser sources are modulated so that the resultant intensities along each trace 46 vary according to the picture being plotted, the relative timing of modulation among the various sources being adjusted so that resultant features of the picture become properly aligned on the film. Angle .alpha. is chosen so that the pitch of the traces, p', assumes a desired value. Ideally, the projected width of each spot, in a direction normal to the trace, is equal to the pitch, p'. In the case of discrete LDs, each LD would be oriented so that the long dimension of each emitting region remains normal to trace direction 44; angle .alpha. and the projection parameters can then be chosen so that trace contiguity is maintained. However, in the case of an LD array device, such as an IALDA, the length axis of each lasing region fixedly coincides with the long axis of the array. As a result, the length axis of each projected spot forms an angle .alpha. with the normal to trace direction 44 and consequently the effective trace width, w', becomes narrower than the actual spot width, w, along its length axis. The relation, again, is w'=w*cos .alpha.. It is noted that the ratio of the effective trace width, w', to the trace pitch, p', is equal to the ratio of the spot width, w, to the array image pitch, p, and thus remains constant regardless of the value of angle .alpha..
The second characteristic, namely the anamorphic beam divergence, may cause a difficulty in designing light efficient projection optics, since the numerical aperture of the objective lens must then be very large. It is noted that usually the length of each projected spot need to be in the range of 10-30 microns and thus--much smaller than the length of each emitting region. This requires the projection optics to effect minification, which further increases the necessary NA on their exit side, if they are to accommodate the entire beam.
the aforementioned difficulties may be overcome by employing anamorphic projection optics, such that have a large numerical aperture in the cross direction only and such that will create an as nearly circular spot image as possible.
One general configuration of such optics is an a focal arrangement, which consists of a collimating objective lens, or group, a focusing lens, or group, and an anamorphic modification. One method of such a modification, employed, for example, in U.S. Pat. Nos. 4,520,471 and 4,932,734, is to interject between the two lens groups (which by themselves are generally spherical) a pair of cylindrical lenses that act as a beam expander, or as a modifier of beam expansion, in one axis. Another method, employed, for example, in U.S. Pat. Nos. 5,541,951 and 5,594,752 (FIGS. 1-3), is to compose the objective assembly out of cylindrical lenses of different powers in each axis. It is noted that in both of the last mentioned patents, it is aimed to produce a single spot out of the multiple light sources and therefore there is an array of objective cylindrical lenses in one axis. It is further noted that in the '951 patent, the order of the cylindrical lenses in the two axes is such as to allow the beams to greatly expand in the cross axis and thus more fill the aperture of the focusing lens. It is also further noted that in the '752 patent (FIGS. 4-5) there is provision for a multiplicity of writing spots, whereby each spot is created by an individual focusing lens (each obtaining a collimated beam as described for FIGS. 1-3).
A different anamorphic configuration is taught in U.S. Pat. No. 5,521,748, which generally addresses the task of illuminating an array of light modulators (LM) by means of a non-addressable LD array (an alternative arrangement for imagesetters, mentioned hereabove, with which the present invention is not concerned). Here a certain optical configuration that includes cylindrical lenses in the longitudinal axis, images all light sources onto all the elements of the LM, while another, single cylindrical lens focuses the light sources onto the LM in the cross axis; there is little concern, if any, with the anamorphic ratio or other relationships between the two axes, since there is no reference to a projected spot shape.
A serious disadvantage of anamorphic configurations of prior art, such as described hereabove, is their relative complexity, which results in undue fabrication costs.
In another type of LD array light projection system of prior art, exemplified by U.S. Pat. No. 5,168,288 and also commonly employed in imagesetters, the light emitted by each source is coupled into a corresponding optical fiber; the other ends of all such fibers are arranged in close proximity along a line and projected together by an objective lens onto the film. Such a system overcomes the difficulties arising from both characteristics mentioned with respect to the lasing region, since essentially the entire beam is coupled into its corresponding fiber and since the light emission from each fiber has an essentially circular cross-section. It, however, shares the cost disadvantage of the first type, described hereabove, in that it is difficult to precisely assemble the fibers into the desired arrangement.
There is thus a widely recognized need for, and it would be highly advantageous to have, an anamorphic optical system that projects light from an individually addressable laser diode array as an array of rounded spots, which is simple and inexpensive to fabricate.